1. Field of the Invention
The present invention relates to a filter used as a noise preventing part of a digital signal processing circuit for use in an office automation apparatus, a television receiver and a video apparatus.
2. Description of the Prior Art
To prevent noise in a digital signal processing circuit, a T-LC type inductor-capacitor (LC) filter having an equivalent circuit shown in FIG. 9 is widely used. Conventionally, the filter circuit is generally composed of discrete parts such as a capacitor and a coil. As a filter of this type, for example a filter of a structure shown in FIG. 1 has been proposed. To form the filter part, one end of each of molded coils 21a and 2lb, each made by pressing a lead frame into a coil is connected to electrode terminals 22a and 22b, respectively, and after the other end of each of the molded coils 21a and 2lb is connected to one terminal of a chip capacitor 23 of an appropriate capacitance, the whole part is covered with a magnetic sheathing substance 25. A filter part using a winding coil 24 shown in FIG. 2 has been proposed. This filter part is the same as the above-described filter part in that it is covered with the magnetic sheathing substance after a plurality of single parts are mounted.
The overall sizes of the filter parts shown in FIGS. 1 and 2 depend on the sizes of the coils. However, it is difficult to mold a minute coil and to form a small winding coil, and it is difficult to reduce the size of the filter formed by covering these coils with the magnetic sheathing substance. It is particularly difficult to reduce the thickness. This prevents the improvement of the packaging density. Further, since a plurality of discrete parts are mounted, the gap among the parts increases, so that the electrical loss of alternating currents in the high-frequency band increases.
A noise filter the making of which is simplified by forming a coil and a capacitor on an insulating substrate by thick film printing and using the electrode pattern of the coil also as the electrode pattern of the capacitor is disclosed in Japanese Laid-open Utility Model Application H4-23321.
In this noise filter, however, since the electrode pattern of the coil is also used as the electrode pattern of the capacitor, the capacitance of the capacitor naturally depends on the inductance of the coil. As a result, the capacitance of the capacitor and the inductance of the coil cannot separately be set.
On the other hand, by simply forming a winding coil on the insulating substrate, an inductance sufficient for a filter circuit cannot be obtained.